I'm busy catching up with C++ and reading some ebooks so I can better understand the code in OpenMW, I've also been scanning through it to see how it all works, I'm interested in helping to add more of the MWSE functionality to OpenMW,
Documentation would be of immense help, sometimes I'm at a loss for what a piece of code is doing.
The source code could be more thoroughly commented.

Not sure what the benefit would be. There isn't any place in the source code where we could put this. We would have to add additional "source" files just for the documentation. Of course there is then the problem that these files would be a lot less usable then the wiki (missing hyper-links and such). We would at least need a way to automatically generate/update documentation from the source and also host it somewhere. I am not opposed to such a solution but that would require additional work.

You wouldn't put that kind of documentation into source files, that would be silly. You would just write plain old ReST documents which is similar to Wiki syntax. These files would exist under ./docs/ and sphinx would then render them into whatever format you want. They would still be readable as ReST (which is the point). The hyperlinks would remain, just point to other documents in the ReST files and sphinx will take care of the linkage when it is rendered to html or pdf. We can also link to pages in the Wiki.

Github has a hook for readthedocs.org, it will auto-generate the documents at every tag (release) including merges into master (if we want).

I've used this system for many (widely used) projects now, so it isn't nothing new.

The additional work will likely be on my end. The rest would be up to those that want to document, but at least there would be examples and placeholders in place.

Anyone that wants to update the docs would just get a github account, fork OpenMW and send PR for review just like code reviews. That way more eyes are on it to correct errors and simplifies everyone's workflow because everyone is now using the same tools.

Just a side note to broaden options.
For general documentation I'd recomend Asciidoctor, it can generate HTML/pdf/pub and other formats.
It also is supported directly in github like Markdown, so you can just post the source files in a repo and view them directly.
For instance, this repo https://github.com/asciidoctor/asciidoctor.org can be seen and edited directly online, but it also gets rendered into the site http://asciidoctor.org/ on every PR.
Of course, the rendering uses github pages and the official renderer with additional options to modify CSS and other things, but it provides quite a lot with little investment.

My opinions are my own. I am just a thankful user of OpenMW with no affiliation with the dev team.